Honestly a lot of different reasons. One they can put natgas on a private disconnected grid direct on the same property.

The other big ones is states ignoring the environmental rules of all the negative side effects of said generation.

DC's better be careful or any government change and their choices now will bite them in the ass.

Physical footprint fits in limited available space, well understood turnkey commercial off the shelf solution, and lack of grid scale flexibility/redundancy to best exploit available capacity at different times of day/year.

I think this is a reasonable question -- not a simple deflection. To get more sustainable builds (that people/voters can accept), I think we need a compromise for new major data centers (~250MW+). If you want to build a new major data center, it needs to paired with power production that is near zero carbon. Example: Solar + battery + wind + nat-gas peaker (rarely used). If not possible to build the required solar farm near the data center, then electrical grid interconnect upgrades need to be "fairly" shared (how is debatable).

Because they don’t want to wait for a grid connection that can take years and they need backup generation capacity anyways.

You’ll see them all connecting to the grid in the following years simply because it’s cheaper

Because behind-the-meter generation bypasses many municipal permitting processes and now also skips NEPA review. As diesel keeps climbing it'll get more expensive to truck fuel in and you'll start to see operators begging for the grid connections they scoffed at before.